Hitachi to HS6 carb conversion & rebuild

Farmer42

Club Member
Hi

I have recently swapped my Datsun Hitachi carbs to British HS6 SU carbs and I thought it may be worthwhile posting up some tips and pics when refurbishing them to the correct Datsun spec.

I have attached a Word document to talk it through. I am happy to try to answer any questions although I am no technical expert and relied very heavily on a very helpful guy at Burlen Fuel Systems.

Happy ReadingEXTRA:)

Paul
 

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  • SU Carb rebuild.docx
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AliK

Vehicle Dating Officer
Staff member
Club Member
Really great write up - we should make this a Sticky as I'm sure it will be asked / searched for in the future.

Nice Job Paul! [emoji106][emoji106]

Ps. You paid a heck of a lot less than my ZT carbs, in fact less than half and being bigger, yours probably breath more easily / better too!
 

Farmer42

Club Member
Thanks guys.

Forgot to mention a couple of things.

Firstly, HS6 carbs have a pipe that the crankcase breather hose on British cars attaches to (one on each carb). It is not needed on a Zed as the breather pipe attaches straight to the inlet manifold. You therefore need to blank them off otherwise it will suck air in and the car will run erratically. You can buy pipe caps (car builder solutions) or you can use a piece of fuel hose and clamp a bolt in the end (bodge!!:EXTRAp).

Secondly, British SU carbs run on a fuel pressure of about 2.5 - 3 PSI which I believe is lower than Datsun Hitachis. If the float chamber floods, you will get fuel squirting out of a breather hole under the fuel inlet pipe on the float lid. If this happens, the pressure is too high and is overwhelming the float valve (they are delicate little things and don't like to feel pressured!!EXTRA:D). It will probably happen if you are running an electric fuel pump especially a Facet red Top. I had this and got over it by putting a fuel regulator in the delivery line.

Thirdly, If you do buy new float lids or have stay-up floats, you will need to set the float level. You do this by turning the float lid upside down and simply bending the metal tang as near to the float as possible. The gap between top surface of the float and the float lid is 1/8 inch or 3.2mm. Easiest way is to use a drill bit and lie it across the lid and adjust the float until it touches the drill.

Finally, if you do need the conversion collar to convert the needle from variable or biased needle to the fixed jet needle, the part number is ABF193.
 

dav118118

Club Member
I would think 2.5 psi is ideal for early dome top hitachis? I have a pressure regulator and have it set at 2 psi and seems fine. Does anyone know the correct pressure? Admittedly I haven’t been redlining the car so can’t comment on lack of fuel at the top end
 

jonbills

Membership Secretary
Site Administrator
Yes it’s 1.5 to about 3.5 for SU and Hitachi (and Weber and Dellorto and...) the picture is confused by US forums’ assertions that carbs run 5 psi. That’s Holley’s spec, and normally SU/Hitachi/Weber/Dellorto tolerate it so it goes unchallenged.
 

Farmer42

Club Member
Hi all.

My attention has been drawn to a typo in my write up. The part number for the piston spring is AUC1170 not AUD.

Apologies
 

toopy

Club Member
if you do need the conversion collar to convert the needle from variable or biased needle to the fixed jet needle, the part number is ABF193.

I'm in the middle of renovating my HS6 SU's and i needed to convert from biased to fix needles, so i can continue to use my SM needles currently in the Hitachi's.

However i was completely unaware until today, that there is a slightly different jet bearing (and nut) for the 2 types of needle, if it's a biased needle then the jet and bearing are effectively fixed in a central position when tight. If the needle is a fixed type, the different bearing apparently allows the jet to move slightly when loose, so it and the needle (when fixed to the piston and in position) can be centered and then tightened fully in the correct alignment.
Ive not actually received the parts yet, so can't do a trial fit, but the above is my understanding of the differences after looking through the parts manual/guide. o_O
 

Farmer42

Club Member
I can't recall changing the adjuster nut and bearing or collar when I changed from biased to fixed needles. Maybe I just got lucky & it all lined up OK. I could be wrong and was provided with new ones in the kit but forgot that I fitted them. Might be worth giving the tech dept at Burlen a call to ask. They are pretty good with the advice.
 

toopy

Club Member
Forgot to update on the jet bearing issue above, i emailed Burlen and they confirmed that, yes you should swap to the correct bearing for your needle.
 

Farmer42

Club Member
Just realised that I must have changed all the bearings and adjuster nuts as part of the kit. For some reason I kept all the old bits and in amongst them were the old adjuster parts. Senility must be creeping in as I can't remember doing it.

I'm surprised you used your old SM needles from your Hitachis. I would have thought they would be worn and may give you erratic running with new jets. They are not expensive so worth renewing along with the rest. Are you not tempted to start experimenting with different needles like Ali?
 

toopy

Club Member
The SM needles in the Hitachi's were new and have barely covered 1000 miles,

experimenting with different needles is something in theory i would like to do, but in practice i have no way to measure AFR, besides i like to understand how and why stuff does or doesn't work and Ali's thread on the subject is very interesting, but faffing about for ages hunting for perfection has never appealed, in the ball park fit and forget for carbs is ok for me, at least for now. If the engine gets tweaked down the line for more BHP then i would be more involved with the whole carb needle thing! EXTRA;)
 

AliK

Vehicle Dating Officer
Staff member
Club Member
Thanks dude - without a rolling road and gas analyser, the afr meter and road work IMHO is the only way to get it properly right. But as you say, if perfection isn’t the end game and your engine isn’t modified, then if you get the SMs to work well at idle they should be OK for the rest of the range. They tend to run a little rich but it’s better that way than the other.

The N27 needles that come as stock in 240 carbs would be way too lean at the top end.

What I have learned in all my faffing about is that you can feel the mix being right with your right foot and butt dyno. Namely that the car feels keen on the transition from off (or steady) to WOT. You also feel the real push when the engine comes on song.

Sometimes you have to sacrifice elsewhere, e.g. having a slightly richer idle that you’d want.
 
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